Jimmy Ortiz, 8, holds a candle during a candlelight vigil after watching the nationally televised announcement of US President Barack Obama on immigration reform.
Photograph: Ringo Chiu/AFP/Getty Images

Barack Obama used a heartfelt televised address to the nation on Thursday to explain his decision to enact sweeping immigration reforms that will shield from deportation almost five million people currently living in the country illegally.

In an emotional broadcast from the White House, the president unveiled controversial executive action that will make millions of undocumented migrants eligible to live and work in what Obama described as “a nation of immigrants”.

He urged America to show compassion to newcomers who entered the country illegally but have worked hard and put down roots yet still “see little option but to remain in the shadows or risk their families being torn apart”.

“Are we a nation that tolerates the hypocrisy of a system where workers who pick our fruit and make our beds never have a chance to get right with the law?” he asked. “Are we a nation that accepts the cruelty of ripping children from their parents’ arms?”

The address was a passionate and unapologetic attempt by the president to explain one of the boldest and most contentious decisions of his six-year presidency.

Obama’s actions have infuriated Republicans who argue that he is overstepping his powers to enact what amounts to an ‘amnesty’ to illegal immigrants.

Unless major immigration legislation is passed before 2016, Obama’s decision almost certainly means immigration will be a central issue for candidates in the next presidential election.

It is an especially toxic issue for Republicans, who are united in opposition to Obama’s action but bitterly divided over how to deal with the millions of undocumented migrants in the country. Leaders have said that not acting risks the party’s long term future, but the conservative base has consistently opposed any reform that includes a path to citizenship for those who enter the country illegally.

Furious Republicans equate Obama’s decision to an “amnesty” for undocumented migrants, and are planning measures to counter it when they assume control of both houses of Congress in January.

“If President Obama acts in defiance of the people and imposes his will on the country, Congress will act,” the incoming Republican Senate majority leader, Mitch McConnell, said on the eve of the president’s remarks.

“We’re considering a variety of actions. But make no mistake, when the newly elected representatives of the people take their seats, they will act.”

Obama’s action combines increased resources for border security and a direction to the Department of Homeland Security, which oversees border and immigration issues, to adopt a policy of removing “felons, not families”.

But the most far-reaching aspect of the executive action is the creation of a new “deferred action” program that will benefit the estimated 3.7 million undocumented immigrants who are the parents of US citizens or permanent legal residents.

Those who have been in the country for more than five years, pass a criminal background check, pay taxes and submit biometric data will receive deportation relief and can apply to work.

Unlike the previous order, which applied only to young people of a certain age brought by their parents before 2007, the new DACA program will be expanded to apply to all undocumented migrants, regardless of age, who were brought to the country as children illegally before 2010.

The White House is bracing itself for a political storm over the wisdom, legality and fairness of the president’s decision, which will inflame an already hostile relationship between Obama and congressional Republicans.

The president’s critics immediately said they would challenge the his actions, which they characterised as undemocratic and possibly unlawful.

Republicans have pointed out that these executive actions run counter to dozens of statements by Obama in recent years that appeared to indicate he did not believe he had the power to make such sweeping changes to the immigration system.

The White House was careful to stress Obama was “acting within his legal authority” and made public a detailed legal opinion from the Justice Department’s Office of Legal Counsel, which advised the president on the legality of his decision.

In his address, Obama also challenged Republicans who are unhappy with his moves to bring legislation to replace them, while also denying that his actions are equivalent to an amnesty. “Amnesty is the immigration system we have today – millions of people who live here without paying their taxes or playing by the rules, while politicians use the issue to scare people and whip up votes at election time,” he said.

Mass amnesty would be unfair. Mass deportation would be both impossible and contrary to our character.

Obama added: “Mass amnesty would be unfair. Mass deportation would be both impossible and contrary to our character. What I’m describing is accountability – a common-sense, middle-ground approach: if you meet the criteria, you can come out of the shadows and get right with the law.”

Although several presidents, including Republicans Ronald Reagan and George HW Bush, have enacted executive changes to the immigration system, none have acted unilaterally to shield so many people from deportation.

Almost half of the estimated 11 million undocumented migrants living in the US illegally could benefit from the changes should they apply.

The schemes protecting undocumented migrants from deportation will not apply to recent or future illegal immigrants. Those who do qualify will not receive a path to citizenship, be permitted to leave and re-enter the country or obtain subsidies under the Affordable Care Act.

And the protections will only last for three years and could be reversed by Obama’s successor in the White House. If the order is rescinded by the next president or simply allowed to expire, millions who have signed up, declaring their presence in the country, could theoretically become vulnerable to deportation again.

Hillary Clinton, the most prominent Democratic candidate for the White House in 2016, issued a statement in support of Obama’s action. She said it was an “abdication of responsibility” by Republicans to reform the immigration system that had forced the president to bring the stop-gap measure but added “only Congress can finish the job by passing permanent bipartisan reform”.

Polls indicate voters are divided over how to treat the millions – many of them Latino – living in the country illegally, and Obama used his address to attempt to persuade the country of the benefits of allowing them to stay.

The president said “tracking down, rounding up and deporting millions” was unrealistic, pointing out that illegal border crossings are at a historic low and made the economic case for allowing undocumented migrants the chance to work and pay taxes.

In the most emotional segment of his address, Obama’s voice strained as he argued that Americans “are and always will be a nation of immigrants”.

“We were strangers once, too,” he said. “And whether our forebears were strangers who crossed the Atlantic, or the Pacific, or the Rio Grande, we are here only because this country welcomed them in, and taught them that to be an American is about something more than what we look like, or what our last names are, or how we worship.”

Jose Patino, a Dreamer, becomes emotional while watching US President Barack Obama’s national address in Phoenix, Arizona. Photograph: Deanna Dent/Reuters

Obama’s package of measures also includes an increase in resources to the southern border with Mexico, where there was a brief surge of unaccompanied Central American children over the summer.

Additionally, the Obama administration said it would streamline the immigration court process and, in a move that will please Silicon Valley, make it easier for highly-skilled workers, graduates and entrepreneurs to obtain work visas.

On Friday, the president will fly to Las Vegas to sign the measures at Del Sol high school, where he kickstarted the push for comprehensive immigration reform almost two years ago.

Five months after Obama made that speech, the Senate passed a bipartisan bill that, had it become law, would have bolstered border security and provided a path to citizenship for many of the 11 million people living in the country illegally.

While supported by senior Republicans who are desperate to mend the party’s reputation among Hispanic voters, the legislative efforts stalled in the more conservative House of Representatives.

“Had the House of Representatives allowed that kind of a bill a simple yes-or-no vote, it would have passed with support from both parties, and today it would be the law,” Obama said, adding that he would continue to press for a holistic legislative solution.

“But until that happens, there are actions I have the legal authority to take as President – the same kinds of actions taken by Democratic and Republican presidents before me – that will help make our immigration system more fair and more just.”